A standard motor-vehicle door latch is carried on the edge of the door and engages around a bolt or pin projecting from a door post in a latched position of the latch. This latch can be actuated by an inside door handle and by an outside door handle from a latched position in which it retains the bolt to an unlatched position freeing the bolt. In addition the latch is typically provided with locking mechanism that uncouples the outside and often also the inside door handles from the latching mechanism so that in a locked position one or both of these handles is ineffective to operate the door.
The locking mechanism is normally controlled in turn by inside and outside locking elements. The inside locking element is typically a button or lever on the inside of the door. The outside locking element is almost always a rotary lock cylinder comprising a cylindrical outer part seated in a cylindrical housing forming part of the latch assembly or door and a rotary barrel or core rotatable in the outer part to pivot a lever connected via a linkage, e.g. a rod or bowden cable, to the locking mechanism of the latch.
In the typical installation as described in European patent application 0,508,580 of Isaacs the inner end of the cylinder core is formed as a noncylindrical shaft rotatable about an axis and fitted to a complementary hole in a short lever extending radially of the axis. The outer end of the lever in connected via the linkage to the lock mechanism.
For ease of assembly the lever is pivoted on the cylinder housing so that the cylinder itself can be installed and withdrawn without having to hold the lever in place. Even with this system it is nonetheless fairly inconvenient to install the lock cylinder since the lever normally extends horizontally in its normal unactuated position. Thus to install the lock cylinder it is necessary to move the lever into the horizontal position, holding it from inside the door, while inserting the cylinder in the door housing from outside the door. At best this operation takes some experience and two hands, at worst it actually requires two installers.
German patent 4,312,573 of Sajfert describes a motor-vehicle door latch which is provided with structure that prevents its parts from moving prior to installation. A holding screw secures the parts relative to each other prior to installation and can be moved out of the way after installation to free them up. While this system does prevent the latch parts from assuming an unwanted position prior to installation, it nonetheless adds a step--manually releasing it--to the assembly process so does not create a net saving of steps or time over the prior-art system.